Single list of suported usb devices

@dinhviethoa
CC: @sunglim

Eliminate the duplicate list of supported USB devices in harness_push.dart: instead, programmatically read the already present list from the manifest.

Also add a `description` field for each device listed in the manifest. This is probably an undocumented usage of the manifest, as this field is not explicitly listed in the docs, however this is currently not flagged as an error, and it's the only way to preserve some kind of annotation for a device with the list preserved only in the manifest.